Galactic distances

"js" wrote in message
...
Astronomers can point the Hubble Space telescope a deep space to
find
galaxies at least 15 billion light years distant, in any direction. I
infer from this that the universe is at least 30 billion years old, since
you can point the telescope one way then swing it 180 degrees and point it
the other way. Total distance ( time ) is 15 + 15 billion.

Yet, most cosmologist say the universe is about 15 to 20 billion years
old.

aloha

It's the same 15 billion years (well, more like 13.5 billion years)
in all directions.

"js" wrote in message
...
Astronomers can point the Hubble Space telescope a deep space to
find
galaxies at least 15 billion light years distant, in any direction. I
infer from this that the universe is at least 30 billion years old, since
you can point the telescope one way then swing it 180 degrees and point it
the other way. Total distance ( time ) is 15 + 15 billion.

Yet, most cosmologist say the universe is about 15 to 20 billion years
old.

The HST can "see" about 15 billion light years out. If we point the HST in
any direction, we can see 15 billion in every direction. (I think the actual
number is really a bit smaller) This means our observable universe is 15
billion light years old. Potentially and probably the universe is older,
larger then we know.

BV.

nope you are still looking back 15by. This just defines a "sphere"
with radius 15by that represents for now the farthest back that we can
see. We assume that we are not located in a "special" location in the
universe and so we expect that any other observer anywhere else in the
universe taking a measurement "now" would also be at the center of a
sphere of radius 15by light years.

js wrote:

Astronomers can point the Hubble Space telescope a deep space to find
galaxies at least 15 billion light years distant, in any direction. I
infer from this that the universe is at least 30 billion years old, since
you can point the telescope one way then swing it 180 degrees and point it
the other way. Total distance ( time ) is 15 + 15 billion.

Yet, most cosmologist say the universe is about 15 to 20 billion years
old.

aloha

JS,

The phrase you want to look up is 'observable universe'. It appears to the
earth centered observer that our scopes can look back in time about 14.7
billion years or so. But you should not assume that 14.7 is a radius that
could be doubled to infer a diameter of 29.4 billion years.

The estimable John Dobson would tell you how wrong that assumption is. It
has to do with approaching the speed of light, and other things involving
physics that this stargazer leaves to others. Here 'ya go:

"The observable Universe has a border, some fifteen billion light years
distant in all directions, imposed on us by what is called "the expansion."
It is imposed on the observer by the fact that all the distant objects
appear to be moving away. At some fifteen billion light years from us (at
the present apparent rate of expansion), they are estimated to be receding
at the speed of light. It is this apparent" expansion" that imposes a border
to the observable Universe because things receding faster than the speed of
light are not observable. And if the rate of expansion were increased, the
border would of course be closer."

excerpted from:
http://www.johndobson.org/articles/by/entropy.html

Bob Doyle
Stargazing from SoCal





"js" wrote in message
...
Astronomers can point the Hubble Space telescope a deep space to
find
galaxies at least 15 billion light years distant, in any direction. I
infer from this that the universe is at least 30 billion years old, since
you can point the telescope one way then swing it 180 degrees and point it
the other way. Total distance ( time ) is 15 + 15 billion.

Yet, most cosmologist say the universe is about 15 to 20 billion years
old.

aloha